Allow attaching to dns_adb which is shutting down

The dns__adb_attach() had an assertion failure that prevented to attach to dns_adb if the dns_adb was shutting down. There was a race between checking for .exiting in dns_adb_createfind and creating new_adbfind() - other thread could have set the .exiting to true between the check.

Remove the assertion failure and allow attaching to dns_adb even while shutting down. The process of dns_adb shutting down would be noticed only a moments later when any other callback is called.

Closes #3298 (closed)